This invention relates generally to thin-film transistor (TFT) processes and fabrication and, more particularly, to a TFT polycrystalline film, and method of rapid thermal annealing (RTA) a film of amorphous silicon with the use of a metal absorptive film to absorb radiated energy, conduct heat, and so encourage the crystallization of the silicon film.
The demand for smaller electronic consumer products with higher resolution displays, and larger displays made without degradation of resolution, spurs continued research and development in the area of liquid crystal displays (LCDs). The size of LCDs can be decreased by incorporating the large scale integration (LSI) and very large scale integration (VLSI) driver circuits, presently on the periphery of LCDs, into the LCD itself. The elimination of externally located driving circuits and transistors will reduce product size, process complexity, a number of process steps, and ultimately the price of the product in which the LCD is mounted.
The primary component of the LCD, and the component that must be enhanced for further LCD improvements to occur, is the thin-film transistor (TFT). TFTs are typically fabricated on a transparent substrate such as quartz, glass, or even plastic. TFTs are almost exclusively used as switches to allow the various pixels of the LCD to be charged in response to the driver circuits. TFT performance will be improved, and driver circuit functions incorporated into TFTs, by increasing the electron mobility in the TFT devices. Increasing the electron mobility of a transistor results in a transistor having faster switching speeds. Improved TFTs having increased electron mobility yield smaller, higher resolution, LCD screens, with lower power consumption and faster transistor response times. Further LCD resolution enhancements will require that the TFTs mounted on the transparent substrates have electron mobility characteristics rivaling IC driver circuits currently mounted along the edges of the screen. That is, display and driver TFT located across the entire display must operate at substantially the same level of performance.
The carrier mobility of typical thin-film transistors, with active areas formed from amorphous film, is poor, on the order of 0.1 to 0.2 cm.sup.2 /V.multidot.s. Carrier mobility is improved by using crystallized silicon. Single crystal silicon transistors, which are usually used in TFT driver circuits, have electron mobilities on the order of 500 to 700 cm.sup.2 /V.multidot.s. Polycrystalline silicon transistor performance is between the two extremes, having mobilities on the order of 10 to 400 cm.sup.2 /V.multidot.s. Thin-film transistors having mobilities greater than 100 cm.sup.2 /V.multidot.s would probably be useful in replacing LCD periphery mounted driver circuitry. However, it has been difficult to produce polycrystalline TFTs with electron mobilities of even 40 to 50 cm.sup.2 /V.multidot.s.
Single crystal silicon films, for use with LCDs, are difficult to fabricate when adhered to relatively fragile transparent substrates. A quartz substrate is able to withstand high process temperatures, but it is expensive. Glass is inexpensive, but is easily deformed when exposed to temperatures above 600.degree. C. for substantial lengths of time. Even the fabrication of polycrystalline silicon transistors has been very difficult due to the necessity of using low temperature crystalline processes when glass is involved. Current polycrystalization processes typically require annealing times of approximately 24 hours, at 600.degree. C., to produce TFTs having a mobility of approximately 30-50 cm.sup.2 /V.multidot.s. These processes are not especially cost effective due to the long process times, and the TFTs produces are not suitable for LCD driver circuits.
The direct deposition of amorphous silicon film is probably the cheapest method of fabricating TFTs. Typically, the transparent substrate is mounted on a heated susceptor. The transparent substrate is exposed to gases which include elements of silicon and hydrogen. The gases decompose to leave solid phased silicon on the substrate. In a plasma-enhanced chemical vapor deposition (PECVD) system, the decomposition of source gases is assisted with the use of radio frequency (RF) energy. A low-pressure (LPCVD), or ultra-high vacuum (UHV-CVD), system pyrolytically decomposes the source gases at low pressures. In a photo-CVD system the decomposition of source gases is assisted with photon energy. In a high-density plasma CVD system high-density plasma sources, such as inductively coupled plasma and helicon sources, are used. In a hot wire CVD system the production of activated hydrogen atoms may lead to the decomposition of the source gases. However, TFTs made from direct deposition have poor performance characteristics, with mobilities on the order of 1 to 10 cm.sup.2 /V.multidot.s.
Various annealing methods exist for turning amorphous silicon into polycrystalline silicon. Solid phase crystallization (SPC) is a popular method of crystallizing silicon. In this process, amorphous silicon is exposed to heat approaching 600.degree. C. for a period of at least several hours. Typically, large batches of LCD substrates are processed in a furnace having a resistive heater source. TFTs made from this crystallization process are more expensive than those made from direct deposition, but have mobilities on the order of 50 cm.sup.2 /V.multidot.s.
A rapid thermal anneal (RTA) uses a higher temperature but for very short durations of time. Typically, the substrate is subjected to temperatures approaching 700 or 800.degree. C. during the RTA, however, the annealing process occurs relatively quickly, in minutes or seconds. Glass substrates remain unharmed due to the short exposure time. Because the process is so rapid, it is economical to process the substrates serially. Single substrates can also be brought up to annealing temperatures faster than large batches of substrates. A tungsten-halogen, or Xe Arc, heat lamp is often used as the RTA heat source.
An excimer laser crystallization (ELC) process has also been used with some success in annealing amorphous silicon. The laser allows areas of the amorphous film to be exposed to very high temperatures for very short periods of time. Theoretically, this offers the possibility of annealing the amorphous silicon at its optimum temperature without degrading the transparent substrate upon which it is mounted. However, use of this method has been limited by the lack of control over some of the process steps. Typically, the aperture size of the laser is relatively small. The aperture size, power of the laser, and the thickness of the film may require multiple laser passes, or shots, to finally anneal the silicon. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the annealing process. Further, the wafers must be annealed serially, instead of in a furnace in batches. Although mobilities of over 100 cm.sup.2 /V.multidot.s are obtainable, TFTs made by this method are significantly more expensive than those made by direct deposition or SPC.
It is generally acknowledged that the RTA process of forming polycrystalline silicon would be very effective in fabricating TFT devices in the LCD industry. However, means must be developed to address the contradictory problems of exercising greater control over the temperature of the amorphous silicon films, and decreasing the annealing times. These problems are complicated by the fact that a silicon film must be relatively thick before it is able to absorb a significant percentage of energy radiated during annealment. Typically, TFTs are fabricated by deposited amorphous silicon over layers of oxide material and a transparent substrate. Of these three materials, only the amorphous silicon is able to absorb a significant percentage of light energy. However, as the thickness of the amorphous silicon layers decrease, to improve transistor parameters such as switching speed, the silicon becomes unable to absorb enough light to rapidly anneal. Currently, the TFT industry is attempting to fabricate transistors with active areas as thin as 300 .ANG.. At such a thickness amorphous silicon is mostly transmissive of the light wavelengths typically used for annealment.
Several solutions have been attempted to address the problem of directing additional heat to the thin silicon layers. One solution is to form a thick amorphous silicon film, with an intervening oxide or nitride barrier layer, over the thin silicon film to be crystallized. This method allows enough heat to be absorbed to support RTA, but the additional process steps are burdensome in the cost conscious environment of TFT fabrication. The thick silicon absorption and barrier layers must be removed in a dry etch process, such as plasma etching. This method of etching adds a major process step to in the annealing of silicon.
A further problem in the use of thick silicon layers is the potential of cracks occurring in these layers during the annealment process. As a result of the cracking, the delivery of heat to the intended thin layer of amorphous silicon is non-homogenous. That is, the non-uniform delivery of heat results in poor crystallization, with poor electron mobility. Further, the irregular, cracked pattern in the silicon surface can be transferred to the intended silicon film during etching.
Specific wavelengths of light have been used to encourage the absorption of light in thin films of silicon. Silicon has a higher absorption coefficient at shorter wavelengths of visible light. The tungsten-halogen lamp, generally used in the industry for RTA processes, has a lower intensity at these short wavelengths. In typical IC processes, where relatively thick wafers are used, the tungsten-halogen is sufficient to rapidly heat the substrates in fabrication. A Xe arc lamp has a greater intensity at the shorter wavelengths absorbed by thin films of silicon. However, these lamps are more expensive, and less robust, so that their use adds costs and complications to high volume annealment processes. As evidence of this statement, it is estimated that approximately 90% of the lamps used for annealing in the industry are tungsten-halogen.
Ino and Tani, U.S. Pat. No. 5,302,230, disclose the use of a support plate to encourage the crystallization of silicon. However, the use of a support plate adds complications to a commercial IC process, and the temperatures disclosed in the patent would likely deform glass substrates such as Corning 1737, which is susceptible to damage at temperatures above approximately 600.degree. C.
It would be advantageous if a method were found of rapidly thermally annealing amorphous silicon to form polycrystalline TFT transistors on glass substrates with electron mobilities exceeding 100 cm.sup.2 /V.multidot.s.
It would be advantageous if a method were found for increasing the amount of radiated energy absorbed by the thin film silicon substrate, to fabricate a high quality polycrystalline film suitable for TFTs with high electron mobility and low leakage currents.
It would be advantageous if a method were found to increase to conduction of heat to thin films of amorphous silicon when absorptive layers are used to aid the RTA process.
It would be advantageous if a method were found to decrease the amount of time needed to rapid thermal anneal a thin film of amorphous silicon on a transparent substrate.
It would be advantageous if a method were developed to use the conventional tungsten-halogen heat lamp, or any available heat lamp, to RTA a thin film of amorphous silicon.
Accordingly, a method for crystallizing a thin amorphous film, such as in the fabrication of a TFT, has been provided comprising the steps of:
a) depositing a layer of the amorphous film having a first thickness; PA1 b) depositing a layer of metal absorptive film, having a second thickness, overlying the amorphous film layer, to absorb and conduct radiated energy; and PA1 c) rapid thermal annealing (RTA) to convert the amorphous film layer deposited in Step a) into a polycrystalline film layer, whereby the dissipation of heat through the metal absorptive film layer controls the crystallization process.
The amorphous film is selected from the group consisting of silicon, germanium, and silicon-germanium alloys, although the crystallization of silicon is generally preferred. The metal absorptive film is selected from the group consisting of Ti, Ta, W, TiN, Mo, Nb, V, and Ti--W, and has a thickness in the range between 250 and 5000 .ANG..
The rapid thermal annealing occurs at a temperature in the range between 600 and 800.degree. C. for a period of time between 1 second and 5 minutes. Typically, the amorphous film is doped before the RTA. The dopant is either activated in a separate annealing process, or during RTA.
In some aspects of the invention a layer of oxide film is deposited over the amorphous film layer to act as a barrier between the amorphous film and the subsequently deposited metal absorptive film so that the amorphous film does not react with the metal absorptive film during the RTA.
In a preferred embodiment of the invention, the surface of the metal absorptive film layer is prepared, in a separate process step, to be highly absorptive of radiated energy. The surface of the metal absorptive film layer is prepared by oxidation and anodization, either of which forms an oxide layer over the metal film, or by nitridation, to form a nitride layer over the metal film. In this manner, over 50% of the radiated light, over a spectrum of wavelengths between 200 nanometers and 1 micron, is absorbed by the metal absorptive film. The amorphous film is patterned to form isolated regions either before, or after the RTA process.
A thin-film structure for use in a thin-film transistor (TFT) of an LCD, made in accordance with the above-described method, is also provided. The substrate comprises a transparent substrate and a polycrystalline silicon film overlying the transparent substrate. The polycrystalhine silicon film is formed by rapid thermal annealing (RTA) an amorphous silicon film, having a first thickness, with a metal absorptive film layer, having a second thickness, temporarily overlying the amorphous silicon film. The temporary metal absorptive film layer is deposited to absorb and conduct radiated energy to said amorphous silicon film, controlling the crystallization process. The temporary metal absorptive film is removed in subsequent process steps. In this manner, thin layers of amorphous silicon are heated sufficiently to rapid thermal anneal said amorphous silicon film with a heat lamp.